From what i can tell there are 2 (3) ways to get Air Mastery
1. a rare mission in the city will let you recruit a air master if you crit but its difficult
2. if you have a air user and some air user slaves a mission will pop up after some time saying you can infuse your slaver with the power of the slaves magic upgrading them while also stripping the slaves of magic, it has a 200 turn timer so you can take your time
(3. debug)